fix(mcp): project multi-content AIContent[] tool results for models

[syf2211]

Summary

Fix MCP tool result formatting when the SDK returns a successful multi-content result as in-memory AIContent[] objects (no structuredContent). The model now sees readable text and MIME markers instead of the literal type name Microsoft.Extensions.AI.AIContent[].

Fixes #2051

Motivation

For successful multi-content tool results (e.g. image + text chart payloads), McpToolResultFormatter.Format() only handled serialized JsonElement CallToolResult shapes. Clean AIContent[] values fell through to ToString(), which surfaces the collection type name and hides all tool output from the model.

Changes

• Handle AIContent, AIContent[], and IEnumerable<AIContent> before the ToString() fallback
• Project TextContent blocks as readable text, joined with newlines in order
• Project DataContent image blocks as [image: <mime>] markers (no base64 bytes)
• Project non-image DataContent as [attachment: <mime>]
• Add regression tests for image+text, image-only, text-only, single TextContent, and non-image attachments

Tests

• dotnet test src/Netclaw.Actors.Tests/Netclaw.Actors.Tests.csproj --filter FullyQualifiedName~McpToolResultFormatterTests — 15 passed
• Reviewed with composer-2.5: APPROVE

Notes

• Existing JsonElement error and structuredContent formatting paths are unchanged
• Image bytes are never embedded in the model-facing string; only MIME markers are shown
• Broader artifact routing policy for non-text MCP results is tracked separately in [Design] Non-text MCP tool result artifacts: modality-aware routing of arbitrary-MIME content to models and users #2052